Many buildings extend one or several floors below ground level. This area provides functional space that can be used for offices, storage units, mechanical rooms, electrical rooms, parking, etc. The main challenge with underground areas is creating an effective enclosure, to avoid problems such as water leaks into basements, poor indoor air quality and high humidity.
These issues are very disruptive to construction operations, but can be avoided with careful analysis and design. Unfortunately, a complete below-grade system analysis is rarely requested and, as a result, underperforming cabinets are common.
Make sure your building design includes adequate insulation and waterproofing.
Designing a Below-Grade Cabinet System
There are two important aspects that must be considered when designing underground systems: the durability of the construction materials and the durability of the project itself. Unlike other building elements that can be renovated many times, a below-grade system cannot be modified easily and must be designed for the entire lifespan of a building:
- Systems below grade are difficult to access and this increases the cost of repairs and modifications. Repairing underground systems can be particularly expensive.
- When considering the materials and design of a below-grade system, focus on the life cycle cost rather than the initial cost.
Some areas below ground level are used as vaults, tunnels or other types of extensions, and waterproofing is crucial for these features. In particular, superior waterproofing is very important when it comes to tunnels. Thermal insulation and drainage are also important factors in the project.
Deterioration of systems below ground level can occur for reasons such as high humidity, thermal effects, weather, heavy traffic, exposure to chemicals, overloading and inadequate material selection. Rapid deterioration can also be a consequence of poor design decisions and poor construction.
Basic concepts of below-level systems
The three main elements that make up a below-grade enclosure are foundation walls, at-grade slabs, and plaza decks. Together, these elements must fulfill four main functions:
- Structural Support: The below-grade system is considered part of the building structure. Its function includes supporting and transferring loads imposed by the environment and the building itself.
- Environmental control: The below-grade system must control any loads resulting from the separation of the indoor and outdoor environments.
- Finish: The enclosure surfaces are the interfaces with the internal and external environments. These surfaces must meet several quality requirements, including wear resistance.
- Distribution: The below-grade cabinet must interface with building systems such as electrical power distribution, communications, water, gas, HVAC and security.
Waterproofing Principles
A key attribute of an effective below-grade system is the ability to control humidity. Michael T. Kubal, a construction waterproofing expert, states that: “Up to 90% of all water intrusion problems occur within 1% of the building's total area or exterior surface.” This means that attention to detail is critical when designing below-grade enclosures, especially at wall penetrations and connections to adjacent elements.
Some building elements that are often vulnerable to water leaks include tie holes, roof penetrations, door and window frames, and intersections between walls and floors. In general, any interface between building elements that is exposed to the outside environment is a potential problem location for water leaks.
Another important statement from Kubal is that “approximately 99 percent of waterproofing leaks are attributable to causes other than material or system failures.” Logically, this refers to human error, and failures are a typical consequence of inadequate design or construction. The following are some common installation mistakes that can lead to waterproofing problems:
- Incorrect installation of building system components.
- Specify inappropriate components for a given application.
- Lack of planning.
- Not using a suitable primer.
- Contact points between incompatible materials.
- Insufficient curing time for concrete.
Because most leaks occur in a very small area and material failures are rare, the design and construction process is critical to achieving effective waterproofing. However, this also presents an opportunity: professional design and installation can minimize water leakage problems throughout the life of a building. Special attention is required when constructing component interfaces in locations exposed to water.
